Conventional semiconductor type pressure sensors include a sensor chip for sensing an applied pressure and a case for accommodating the sensor chip. The sensor chip includes a semiconductor substrate such as a silicon substrate and has a diaphragm that is deformable in accordance with an applied pressure.
As disclosed in JP-A-H11-14480, a sensor chip of a conventional pressure sensor is fixed to the case via a glass pedestal. The glass pedestal has a through hole for introducing pressure to the diaphragm of the sensor chip, and the sensor chip is mounted thereon by anodic bonding. The glass pedestal is fixed on the case by an adhesive.
Herein, the glass pedestal is composed of Pyrex® glass, for example, in order to adjust the coefficient of thermal expansion thereof to that of the sensor chip, and has a certain height to space the sensor chip from a bonded part between the glass pedestal and the case. By the glass pedestal spacing the sensor chip from the bonded part, a stress such as a thermal stress generated at the bonded part is prevented from reaching the sensor chip.
As described above, the glass pedestal reduces thermal stress that would otherwise affect the sensor chip. Therefore, if the glass pedestal is omitted and the sensor chip is mounted directly on the case, thermal shock, which is generated during and after adhesion at the bonded part, is applied to the sensor chip. This strains the diaphragm of the sensor chip and negatively affects the sensor characteristics.
The above mentioned thermal shock may be composed of various stresses such as a stress applied directly on the sensor chip during adhesion, a stress due to the differences in coefficients of linear thermal expansion among the sensor chip, the case and the adhesive, and a stress due to the creep or plastic deformation of the case.
Furthermore, in case the sensor chip is mounted directly on the case by the adhesive, the adhesive may extend to the diaphragm of the sensor chip and adhere to the diaphragm. This increases the possibility of that the sensor characteristics of the sensor chip are negatively affected. This is because the surface tension of the adhesive used to bond this kind of sensor chip is generally small.